Glass ceramics

ABSTRACT

A high-alumina glass-ceramic product with a substantial proportion microcrystalline is formed from a glass by thermal treatment, the main crystal phase being beta-spodumene or betaeucryptite, and the product having a thermal coefficient of expansion of less than 40 X 10 7/*C. At least 90 percent of the composition comprises major constituents and nucleating agents as follows, in percentages by weight: Major constituents: SiO2 55-75Al2O3 15-30 Li2O 2-15MgO 0-20 Nucleating agents: P2O5, 0.5-6.0 percent, together with one of the following metallic oxides: A. MoO3 0.5-4.0 b. WO3 0.5-4.0 c.V2O5 0.52.0 d. TiO2 0.2-less than 2. The thermal coefficient of expansion is less than 40 X 10 7. An alternative nucleating agent is cadmium sulpho-selenide.

O United States Patent [151 3,

McMillan et a1. 1 1 Mar. 7, 1972 OTHER PUBLICATIONS Handbook of Glass Manufacture 11, Pages 192 to 199. .1. Ceram. Assoc. Japan, No. 69, Mar. 1961, Pages 67, 74. GLASS CERAMICS J. Ceram. Assoc. Japan, No; 70, Jan. 1962,pages 103 to 110.

Megumi Tashiro and Masamichi Wada, two pages, combined Inventor-S1 William McMillan; Graham with Naohiro Soga, Sumio Sakka, and Megumi Tashiro three ridge, both of stafiol'di England pages, making a five page brochure of an address or addresses 73] Assignee: The Englidl Electric company, Limited. mfif f zg Imemamna' Glass wash England The 0138s industry, Pages 76 to 82, Feb. 1964. Notice: The portion of the term of this patent sublaimed, Attorney-Stevens, Davis, Miller & Mosher 22 Filed: Aug. 13, 1969 [57] ABSTRACT [21] PP 849,794 A high-alumina glass-ceramic product with a substantial proportion microcrystalline is formed from'a glass by thermal Related Us'Apphcamm Data treatment, the main crystal phase being beta-spodumene or 63] Continuation of $81. No. 636,267, May 5, 1967, abanbeta-wcrypme and the Product having a thermal coefl'wlem doned, continuatiomimpm f Sen 2 4 2 May of expansion of less than 40X10"/C. At least 90 percent of 24 1966, abandoned continuatiomimpan of Ser the composiltlion comprises major constituents and nucleating 233,371, May 27, 1963, abandoned, Continuation-inagems as percemages by wflght' part of Ser. No. 201,718, June 11, 1962, abandoned. I Major constituents:

[30] Foreign Application Priority Data J 20 1961 o B 22 232/61 55-75 une reat r1ta1n v A1203 l5 75 L' s21 u.s.c1 ..106/39 DV, 65/30, 65/33, a

65/ 134 [51 1 Int. Cl. ..C04b 33/00, C031) 29/00 IIIIIIIIIIIIIIIIIIIIIIIIII Nucleatmg agents: P205, 0.56.0%, together Wlth one Of the following metallic oxides:

[56] Refmm M60 ..0.5 4.0

UNITED STATES PATENTS 3,157,522 11/1964 Stookey ..65/33 X o than 3,117,881 1/1964 Henry et a1.. .....65/33 X 3,238,085 3/1966 Kobe et al ..65/33 X h @gfifigiem'gfjlggsign kjssvthan -1 1 McMillan 5t 31 X An alternative nuc'eating agent i cadmium u!ph e|enide FOREIGN PATENTS 0R APPLICATIONS 8 Claims, 8 Drawing Firures 786,582 1 1/1957 Great Britain ..65/33 sequent to Nov. 14, 1984, has been dis- Primary Examiner-Frank W. Miga 'WT/ P205 CASES(Q) a. (b) CASE CASE (d) PAIENTEDMAR 7 I972 SHEET 1 OF 4 A1 0 WT Li O-Al O -SlO SYSTEM FlG.i

, 'wT/0 P205 cAsEs(J) a (b) CASE (0 CASE (d) FIG. 7

PATENTEDMAR 7 I972 SHEET 1 OF 4 |(qmbmmmvzl22.23.

TIME (HouRs) FIG. 8

GLASS CERAMICS This application is a continuation of application Ser. No. 636,267 filed May 5 1967; that application was a continuation-in-part of application Ser. No. 552,422 filed May 24 1966; and that application was a continuation-in-part of applications Ser. Nos. 283,371 filed May 27 1963, and 201,718 filed June 11 1962; and all now abandoned.

The present invention relates to glass-ceramics.

A glass-cerarnic is a material formed from a glass by heat treatment, the heat treatment being such as to induce crystallization and resulting in a very large number of microscopic crystals being formed. A nucleating agent is included in the glass to aid in the production of the large number of crystals. Such glass-ceramics are in general fairly hard, dense, and strong.

The object of the invention is to produce glass-ceramics having a desirable combination of properties. Among these properties are: a composition in the SiO -Al O -Li O system; a low coefficient of thermal expansion; a high strength; and the absence of any special difficulties or disadvantages such as the need for special atmospheres during manufacture or the-use of poisonous or otherwise undesirable materials.

The glass'compositions used in the present invention have constituents in the following ranges, in percentages by weight:

so, 55-15 A1,0, 15-30 Li,o 2-15 MgO -20.

The nucleating agent used is a combination of two components: the first component is P 0 0.5-6.0 percent by weight, and the second component is one of the following metal oxides:

a. mo, 0.5-4.0 b. wo, 0.5-4.0 c. v,o, 0.5-2.0 d. "no, 0.2-less than 2.

The main crystal phase which results is beta-spodumene or beta-eucryptite.

It has been known to use certain metal oxides, alone or in certain combinations, as nucleants for glasses of the above system. The most common nucleant is TiO However, in U.S. Pat. No. 3,117,881, Henry et al. have proposed the use of ZrO in combination with one of several other oxides as a nucleant. This specification gives the thermal coefficients of expansion of some of the resulting glass-ceramics as from 86 to 200 l0"/C; these values are much higher than those aimed at by the present invention, the lowest being more than twice the upper limit of the range achieved by the present invention. Also, in U.S. Pat. No. 3,238,085, Hayami et al. have proposed the use of As O alone or together with M00 or W0 as a nucleant. The use of arsenic obviously excludes the use of the resulting glass-ceramic for many purposes, e.g., domestic purposes.

It is known that P 0 is an effective nucleant for certain silicate glasses, but it has been found that for glasses with more than, say, percent of A1 0 P 0 is not an effective nucleant. However, it has now been found that P 0 in combination with any of the metal oxides listed under (a) to (d) above, is effective as a nucleant for high-alumina glasses, and further that the resulting glass-ceramics have strengths very considerably greater than those obtainable by the use of metal oxide nucleants alone.

An alternative nucleating agent provided by this invention is cadmium sulpho-selenide.

Several examples of the preparation of glass-ceramics in accordance with the invention will now be given, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a 3-variable diagram showing the ranges of the major constituents in a u,o-A|,o,-sio, system;

FIGS. 2 to 6 are is a series of 3-variable diagrams showing the ranges of the major constituents in a Li O-Al O -SiO MgO system for five different proportions of SiO FIG. 7 is a diagram illustrating proportions of nucleants; and

FIG. 8 is a time-temperature diagram illustrating heat treatments.

A mixture of raw materials to give the desired glass composition and incorporating the desired quantity of nucleating agent is melted at a temperature high enough to produce a bubble-free melt. As shown in FIG. 1 for the case where MgO is absent, and in FIGS. 2 to 6 for the cases where the proportion of $0, has five special values between 55 percent and 75 percent by weight, the major constituents of the glass are, in percentages by weight:

SiO, 55-75 Al,0, 15-30 Li,0 2-15 The nucleating agent consists of 0.5-6.0 percent of P 0 together with one of the following metal oxides:

a. M00, 0.5-4.0 b. W0, 0.5-4.0 C. V O; 0.5-2.0 d. TiO, Ill-less than 2.

The proportions of the possible second nucleants (a) to (d) are plotted in FIG. 7 against that of P 0 the major constituents together with the desired proportion of nucleating agent should total at least percent of the glass. The remaining 10 percent of the glass may be made up of various nonessential constituents some of which take part in the glassy matrix of the glass-ceramic together with residual SiO A1 0 U 0 and MgO. The nature of the optional minor constituents and the amounts in percentages by weight which are permissible are as follows:

i. alkali metal oxides (Na O and K 0) 0-5 either alone or combined,

ii. zinc oxide (ZnO) 0-8.9, iii. calcium oxide (CaO) 0-5,

iv. boric oxide (B 0 0-7.6. The following batch materials may be used for the major constituents: 4

Ground Ouartz SiO, Aluminum Oxide Alp, Aluminum Hydroxide AltOH Lithium Carbonate Li,C0, Magnesium Oxide MgO and for the minor constituents:

Sodium Carbonate Na,C0, Sodium Nitrate NaNO, Potassium Carbonate KLO, Potassium Nitrate KNO, Zinc Oxide ZnO Calcium Carbonate CaCO Bcric Acid H BO The batch materials are thoroughly mixed before melting. As a further constituent of the batch suitable quantities of the nucleating agents are added. These may conveniently be added to the batch in the forms of tungsten trioxide, molybdenum trioxide, vanadium pentoxide and/or titanium dioxide, together with phosphorus pentoxide, but other compounds of tungsten, molybdenum, vanadium and/or titanium together with for example an alkali metal phosphate may be used provided they are compatible with the glass structure. Such compounds include molybdates, tungstates, vanadates, and titanates of alkali metals.

As disclosed in Ser. No. 283,371, the batch material may contain cadmium-sulpho-selenide in an amount such as to give in the glass from 0.5-2.0 percent by weight of cadmiumsulpho-selenide either alone or with another nucleating agent.

The batch mixture is melted in crucibles at a temperature in TABLE I8 the range l,200l ,500 C. depending on composition. The molten glass is then formed into the required shape by a normal glass-working process, such as casting or pressing. Composition NO.

If it is required to store the articles before devitrification 5 they are annealed at a suitable temperature depending on the glass composition. For the controlled devitrification by heat T 6 3': :2: :3; g: 3-: treatment, the temperature of the articles is first raised at a 2 1-, rate not exceeding C. per minute, and preferably at MgO between 3 C. and 5 C. per minute to a nucleation temperal0 K 4 2 4 3 3 4 3 4 3 ture which is preferably between the softening point (mg. :2: 3 1 1 pomt) of the glass and a value 50 C. above the softenmg cs0 C point. The nucleation temperature is usually in the range 1 5 600-700 C. for these glass compositions. 1 5 3 6 1 3' If however it is not required to store the articles, they are L8 M transferred direct, at the temperature which they have v,o, 0.5

reached after working, to a furnace maintained at the nucleation temperature. In either case the articles are maintained at the nucleation temperature for a period which depends on the 20 glass composition and which may be as short as 2 minutes but TABLE [C may be 2 hours or longer. This treatment causes the formation of nuclei dispersed through the glass.

The temperature is then further raised at a rate not exceed- Composition Nov ing 10 C. per minute to the final crystallization temperature, 13. 14. 15. 16. 17. 18. which varies from approximately 700 to l,250 C. depending on the composition, and is usually in the range 950 1,100 C. $10, 64.1 63.6 65.3 64.5 61.9 61.2 This temperature is maintained for a period of not less than 15 I P minutes and preferably for at least 2 hours, again depending a? on the composition, and during this stage the crystallization 5 29 23 occurs and a dense glass-ceramic product containing closely 1 interlocking crystals is obtained. The articles are then allowed i g f to cool at a rate not exceeding 10 C. per minute, the normal I I I I cooling rate of the furnace usually being satisfactory. v.0, 2.5 2.5 1.4 1.4 1.3 1.3

Specific examples of the process will now be described: -1

Mixtures of raw materials to give glasses of each of the com- :72 I f f positions indicated in percentages by weight in Tables IA to [H M 1 were melted in malachite-type crucibles.

The glasses were then shaped to produce the desired article by normal glass-working processes, such as casting or TABLE lD pressing.

Samples of the glass compositions were then heat-treated as shown in Tables "A to "F. The temperature of each sample was raised at a rate not exceeding 10 C. per minute to the 19. 20. 21. 22. 23. 24. temperature spec1fied 1n the first hne of the Tables, and th1s 7 temperature was maintained for the time specified in the SiO, 61.6 62.0 61.2 62.] 60.6 61.!

second lme The temperature was then raised at a rate not ex- M0: H I 8,3 I '8, m3 ceedmg 10 C. per mlnute to the temperature speclfied 1n the up 5.7 5,4 5.5 5.1

third line of the Tables, and this temperature was maintained 5 for the time specified in the fourth line, after which the tem- :f 2 i "f i 3' perature of the samples was reduced at not more than 10 C. Zno 83 To 51 53 51 5'2 per minute to room temperature. FIG. 8 shows the time-tem- Ca perature relationships for the two-stage heat treatment, using 2 l 3 a 4 the compos1t1ons lndlcated on the F1gure. 1 55 M00, H n 3.3 M 13 n TABLE lA v, r10, L8

Composition No. 1. 2. 3. 4. .5. 6.

TABLE [E 510, 61.0 61.7 62.5 62.4 65.1 62.6 41,0, 13.2 18.5 10.5 18.1 19.3 18.7 I up 5.6 5.7 5.7 5.7 5.8 5.7 composmon 8 25. 26. Y 27. 28. 29. 30. 1 ,o 4.1 4.2 4.2 4.2 I 4.5 4.2 116,0 zno 52 53 53 5'3 53 510, 62.4 63.0 63.6 61.2 61.3 62.2 Cao 41,0, 18.2 18.1 18.1 18.4 13.4 18.6 B20: at?) 4.4 4.0 3.5 5.4 5.4 5.4

g 5:36 :1; if: no 4.1 4.0 4.1 1 3.6 2.9 29 wo, 3.5 Niko o5 I x 4 ZnO 5.2 5.2. 5.2 5.7 6.3 5.2 T10, 1.3 C30 13,0, 1 ,0, 2.4 2.4 2.3 2.4 2.4 2.4

20. 2 Main crystal phase in glass-ceramic 1 Beta spodumene.

W() Li,0 4.0 5.7 5.7 5.5 5;8 12.5 v,o, MgO 16.0 T10, K 1.0 213 4.3 4.2 4.3 2.5

Na,O ZnO 3.3 .5.3 5.3 5.3 CaO TABLE 1F M00, 3.0 0.6

-wo, v.0. 06 1.8 1.8

Composition No. 31. a2. as. 34. as. 36.

S10, 61.1 60.7 71.0 55.0 56.0 62.4 AI,O, 18.3 10.3 15.0 26.0 20.0 19.0 Li,0 5.4 5.4 5.0 12.0 12.5 6.0 MgO 1.0 -4.5 K,O 4.2 4.2 3.0 3.0 4.5 Na,0 TABLE "'1 ZnO 5.2 5.2

CaO B101 Composition No. 1 ,0, 2.4 2.4 3.0 4.0 5.5 2.5 41 M 45' M M00 3.3 3.3 3.0 3.0 3.0 11 W0, vgos $0. 790 73.5 73.5 73.5 no: A110,- 4.0 10.0 10.0 10.0 Li,O 12.5 12.5 12.5 12.5

M K,0 2.5 3.5 3.0 2.0 30 Na,0

ZnO

TABLE 1G C80 11,0,

Composition N0. W0a 37. 3s. 39. 40. 41. 42. vo}

T10, 510, 511.0 66.8 62.8 62.1 63.5 110.5 H 2. 0.5 1.0 2.0 A00 15.0 10.9 18.9 18.7 18.8 4.0

TABLE 11A Composition N0.

Heat-treatment:

1st stage:

Temperature, C 650 700 600 650 700 700 700 650 700 700 Time, hours 2 2 2 2 2 2 2 2 2 4 2nd stage:

Temperature, C 1,000 1,00 050 1, 000 1,000 1, 000 1, 050 1,050 1,000 1, 000 Time, hours 2 2 2 2 2 2 2 2 M Properties of glass'eeramic.

Modulus of rupture p.s.i. 10" 16.2 12.5 14. 2 12. 16. 0 12.0 Expanslon coefi. per (3.)(10 (20500 C.) 22. 0 20. 6 25. 0 22.0 23.0 15. 4 32.2 15. 5 13.1 2. 5 Mam crystal phase in glass-ceramic (2) 1 Beta spodumene. Beta eucryptite.

TABLE IIB Composition No.

6(a) (b) 7(a) 7(b) 8 9 10 11 12 13(a) Heat-treatment:

st age:

Temperature, C 650 700 650 700 700 700 700 700 700 700 Time, hours 2 2 2 2 2 2 2 2 2 2 2nd stage:

Temperature, C 1,000 1,000 1,000 1,000 1,000 1,000 1,000 1,000 1,000 1,050 Time, hours 2 2 2 2 2 2 2 2 2 2 Properties of glassceramic:

Modulus of rupture p.s.i.X10' 19.0 13.0 12.0 24.8 14. 5 23.3 22.6 4.6 24.7 16 8 28.0

TABLE I Composition N 0.

Heat-treatment:

1st stage:

Temperature, C 700 700 700 050 700 700 650 700 700 700 Time, hours 20 2 2 2 2 4 2 2 2 1 2nd stage:

Temperature, C 1,000 1,050 1,000 1,050 1,000 1, 000 1,050 1,000 1,000 1,000 Time. hours 14 2 2 2 '2 ,4 2 2 2 2 Properties of glass-ceramic:

Modulus of rupture p.s.i.X1O 11. 5 Expansion coefi. per C. 10" (20-500 0.). 14.2 26.6 24. 2 7. 5 6.7 13.1 5.4 2.2 13.4 14.0 Main crystal phase in glass ceramic 1 Beta eueryptite.

TABLE 11D Composition N o.

Heat-treatment:

1st stage:

Temperature, C 700 650 650 650 650 080 650 080 7 650 Time, hours 2 3 2nd stage:

Temperature, C 1,050 Time, hours 15 Properties of glass-ceramic.

Modulus oi rupture p.s.i. 10 10 Expansion 0006. per C. 10 (IO-500 C.) 1. 2 Main crystal phase in glass-ceramic 1 Betaeucryptite. 7

TABLE IIE v Composition No.

Heat-treatment:

stage:

Temperature, C 700 650 600 700 700 700 700 700 700 700 Time, hours 3 3 3 3 1 1 .1 2 2 2 2nd stage:

Temperature, C 1,075 1,000 1,000 1,075 1,000 1,000 1,000 1,000 1,000 1,000 Time, hours 1 1 1 1 2% 2% 2 2 2 2 Properties of glass-ceramic.

. Modulus of rupture p.s.i. 1[l" 15. 7 18.1 12. 2 13. 4 Expansion coefi. per C.) 10 (Al-500 C.) 20.4 12.0 -7.7 t1. '2). U 10. 5 Main crystal phase in glass-ceramic v 1 Beta oucryptite/quartz solid solutions. 4 2 Beta spodumene/quartz solid solutions.

TABLE substantial proportion is microcrystalline, consisting essentially of a composition having major constituents and nucleat- Composition No.

mg agents wh ch together form at least 90 percent by weight 1 o 1 3 38 30 4 4 of the composltion, said ma or constituents consisting of S10 ggg f M 0 U 0, and optionally MgO within the following ranges Temperature, C 700 700 700 700 700 Percentages y weight Time, hours- 2 2 2 2 2 2nd 'i t C 000 000 1 000 1 000 1 000 empora ure. 1, Time. hours 2 2 2 2 2 2 5 :2; Properties of glass-ceramic.

Modulus of rupture L110 p.s.i.X100 MgO 0-20. Expansion coefl. per C.X

10 (20-500 C.) 21.9 Main crystal phase in and said nucleatmg agents consisting of 0.5- 6.0 percent by glass'ceramm weight of P 0 together with any of the following, in percent- 1 Beta spodumene/quartz solid solutions. ages by weight,

It was found that the samples had been converted into microcrystalline ceramics of good mechanical strengths with m? xiithe moduli of rupture and with the linear thermal expansion v10: coefficients in the range 20-5W C. as given in the fifth and d. TiO, 0.24m than 2.

sixth lines of Tables 11A to "F.

The glass-ceramics thus prepared were found to have finer said product having its main crystal phase selected from the crystalline structures and to be mechanically stronger than group consisting of beta-spodumenc and beta-eucfyPtite, and otherwise similar glass-ceramics using phosphates alone tohaving a coefficient of thermal expansion of less than 40 X serve as nucleating agents. 10".

The ceramic materials resulting from controlled devitrifica- 2. A glass-ceramic product as claimed in claim 1 containing tion in this manner are colored as follows: as optional minor constituents, up to a total of not more than incorporating molybdenum: pale. buff to pale yellow 10 percent by weight, any of the following, in percentages by incorporating tungsten: pale greyish violet ight:

incorporating vanadium: grey to grey brown incorporating titanium: pale violet. I. "no and K10 68cm 0-5 We claim: ii. Z110 g 9 1. A hard, dense, mechanically strong, electrically insulatiii. gag

ing, high-alumina glass-ceramic product of which at least a 3. A glass-ceramic material formed by devitrification of a glass consisting of major constituents which, together with a nucleating agent, make up at least 90 percent by weight of the total composition and lie in the glass system consisting essenal y a so, 454m A110. 040% up o-zm MgO o-am SiO, 55-75 ALO, 15-30 Li,0 2-l5 MgO -20 and said composition containing as nucleating agent the combination of P 0 and 'liO within the following ranges in percentages by weight:

0.5-6.0 0.2-less than 2 said product having its main crystal phase selected from the group consisting of beta-spodumene and beta-eucryptite, and having a coefficient of thermal expansion of less than 25X 5. A glass-ceramic product as claimed in clairnA containing as optional minor constituents, up to a total of not more than 10 percent, any of the following in the stated percentages by weight:

i. Na O and K 0 0 5, but not exceeding 5 percent in combined total, ii. Zn() 0 8.9 iii. CaO 5.0

iv. B 0 7.6.

6. A hard, dense, mechanically strong, high-alumina glassceramic product of which at least a substantial proportion is microcrystalline, consisting essentially of a composition having major constituents and a nucleating agent which together form at least percent of .the composition, said major constituents consisting of Si0=, M 0 Li,0 and optionally MgO within the following ranges in percentages by weight:

SiO, 55-75 ALO, 15-30 Li,O 2-] 5 M30 0-20 and said composition containing as nucleating agent the combination of P 0 and V 0 within the following ranges in percentages by weight:

BO, V,O,

said product having its main crystal phase selected from the group consisting of beta-spodumene and beta-eucryptite, and having a coefficient of thermal expansion of less than 25X 1 0 

2. A glass-ceramic product as claimed in claim 1 containing as optional minor constituents, up to a total of not more than 10 percent by weight, any of the following, in percentages by weight: I. Na2O and K2O together 0-5 ii. ZnO 0-8.9 iii. CaO 0-5.0 iv. B2O30-7.6.
 3. A glass-ceramic material formed by devitrification of a glass consisting of major constituents which, together with a nucleating agent, make up at least 90 percent by weight of the total composition and lie in the glass system consisting essentially of: SiO245-88% Al2O3 0-36% Li2O 0-27% MgO 0-32% optional minor constituents consisting essentially of alkali metal oxides 0-5 percent; zinc oxide 0-8.9 percent; calcium oxide 0-5 percent; and boric oxide 0-10 percent; and a nucleating agent being cadmium-sulpho-selenide 0.5-2.0 percent by weight.
 4. A hard, dense, mechanically strong, high-alumina glass-ceramic product of which at least a substantial proportion is microcrystalline, consisting essentially of a composition having major constituents and a nucleating agent which together form at least 90 percent of the composition, said major constituents consisting of SiO2, Al2O3, Li2O and optionally of MgO within the following ranges in percentages by weight: SiO255-75 Al2O315-30 Li2O 2-15 MgO 0-20 and said composition containing as nucleating agent the combination of P2O5 and TiO2 within the following ranges in percentages by weight: P2O50.5-6.0 TiO20.2-less than 2 said product having its main crystal phase selected from the group consisting of beta-spodumene and beta-eucryptite, and having a coefficient of thermal expansion of less than 25 X 10
 7. 5. A glass-ceramic product as claimed in claim 4 containing as optional minor constituents, up to a total of not more than 10 percent, any of the following in the stated percentages by weight: i. Na2O and K2O 0 - 5, but not exceeding 5 percent in combined total, ii. ZnO 0 - 8.9 iii. CaO - 5.0 iv. B2O3 - - 7.6.
 6. A hard, dense, mechanically strong, high-alumina glass-ceramic product of which at least a substantial proportion is microcrystalline, consisting essentially of a composition having major constituents and a nucleating agent which together form at least 90 percent of the composition, said major constituents consisting of SiO2, Al2O3, Li2O and optionally MgO within the following ranges in percentages by weight: SiO255-75 Al2O315-30 Li2O 2-15 MgO 0-20 and said composition containing as nucleating agent the combination of P2O5 and V2O5 within the following ranges in percentages by weight: P2O50.5-6.0 V2O50.5-2.0 said product having its main crystal phase selected from the group consisting of beta-spodumene and beta-eucryptite, and having a coefficient of thermal expansion of less than 25 X 10
 7. 7. A glass-ceramic product as claimed in claim 1 wherein said TiO2 is present in the range 0.2 - 1.5 percent.
 8. A glass-ceramic product as claimed in claim 4 wherein said TiO2 is present in the range 0 .2 - 1.5 percent. 